The purpose of this research is to study the mechanisms of lactic acid removal in physically trained and normal rats during rest and various levels of exercise. The continuous infusion technique will be applied to anesthetized and unanesthetized resting and exercising animals. Single (U-14C) and dual (U-14C) and (2-3H) labeling techniques will be applied to study the turnovers of glucose and lactate under the above conditions. Pathways of lactate removal will be evaluated by determination of 14CO2 production, blood metabolite specific activity, and quantitative analyses of tissues (muscle, liver, kidney and heart) freeze clamped in situ.